Cities | San Marcos

San Marcos, California Multiplex Rules 2026: Zoning, Lot Sizes, ADUs, SB 9, Three or More Homes, Ridgelines and Hillsides, Permits and Fees

This guide covers the City of San Marcos in San Diego County, California, a city of 94,908 people. A city of the same name in Texas has its own code. Three facts matter most for a project in San Marcos. The R-2 zone permits duplexes and 3-unit or 4-unit buildings that look like a house, at up to 12 homes per gross acre, and the R-3 zones allow 20 or 30. Every attached project in R-2 and R-3 needs a Planning Commission hearing, and an inclusionary housing rule reaches projects of one or more units. And much of the city is on hills, where a ridgeline overlay and slope-based lot sizes limit what can be built. The seven guide pages below cover each topic in depth.

San Marcos Guide Pages

Start With State Law

Five state laws set the minimum San Marcos must allow. SB 9 (2021) allows two main homes on a lot in a single-family zone, a split into two lots, or both, approved by staff with no hearing. ADU law sets the accessory dwelling units (ADUs), second homes on a lot, and junior ADUs a city must allow. SB 684 and SB 1123 allow maps of up to 10 lots with 10 homes. The state density bonus gives extra homes to projects that include affordable homes. And SB 79, the 2025 law on housing near transit, reaches San Diego County: HCD, the state housing department, says in its advisory that its upzoning "will take effect on July 1, 2026, in Alameda, Los Angeles, Sacramento, San Francisco, San Mateo, Santa Clara, and San Diego Counties."

San Marcos writes most of these into its own code. SB 9 two-unit projects are in Chapter 20.470 and SB 9 lot splits in Chapter 19.44, both from Ordinances 2021-1510 and 2021-1511. ADUs are in Chapter 20.410, which opens by saying that where it conflicts with the Government Code, "State Government Code shall prevail." The density bonus is in Chapter 20.305. VanPlex found no local chapter for SB 684 and SB 1123 maps or for SB 79, so those statutes apply on their own terms. San Marcos is a charter city, and the SB 9 statutes define a local agency as a city "whether general law or chartered", so they apply here either way.

San Marcos in Numbers

The US Census Bureau's Vintage 2025 city file estimates 94,908 people in the City of San Marcos on July 1, 2025. The April 2020 estimates base is 94,860, so the city grew by 48 people, or 0.05 percent (VanPlex arithmetic), and the 2025 figure is 616 below the 2024 estimate of 95,524. In that file San Marcos is the 82nd largest city in California, after Manteca (96,693) and ahead of Indio (94,269). San Diego County has 3,282,248 people, so the City holds 2.9 percent of them (VanPlex arithmetic: 94,908 divided by 3,282,248).

The Ways to More Homes Compared

Sources: San Marcos Municipal Code Chapter 20.215, 20.225, 20.305, 20.410, 20.435, 20.470 and 19.44; Government Code 66499.41 and 65915.

Zones That Allow Homes

San Marcos has six residential zones: R-1-20 at 2.0 homes per gross acre, R-1-10 at 4.0, R-1-7.5 at 6.0, R-2 at 12.0, R-3-10 at 20 and R-3-6 at 30. The R-1 zones permit single-family detached homes, R-2 adds attached homes, duplexes and house-like buildings of up to four units, and the R-3 zones permit apartments. MU-1 and MU-2 allow 20 to 45 homes per acre. The code gives three different densities for R-1-7.5. See the zoning districts page.

Lot Sizes and Dividing Land

New lots need 20,000 square feet in R-1-20, 10,000 in R-1-10, 7,500 in R-1-7.5 and 3,500 in R-2 on flat land, and up to one acre on steep slopes. The City Engineer decides maps of four or fewer lots within 50 calendar days, and the Planning Commission decides larger maps. SB 9 lot splits follow Chapter 19.44. See the lot size and subdivisions page.

House Lots: ADUs and SB 9

Chapter 20.410 allows ADUs of 850 to 1,000 square feet, 16 feet tall when detached, with 4-foot side and rear setbacks, decided within 60 days. Chapter 20.470 caps SB 9 units at 800 square feet and 16 feet, and limits a parcel to four units in all. Several City rules are stricter than today's statutes, and the page names each one. See the ADUs and SB 9 page.

Three or More Homes

Multifamily projects need common open space of 30 percent of the livable ground floor area, a tot lot for every 25 units, and 2 parking spaces per unit of two or more bedrooms plus guest spaces. Rental projects of more than six units reserve 15 percent of units as affordable or pay a fee. See the three or more homes page.

Parking, Setbacks and Height

Homes in the R-1 and R-2 zones may be 35 feet or 2 stories. Front setbacks run from 35 feet in R-1-20 to 10 feet in R-2, and roads in the General Plan add setbacks of 77 or 86 feet from the centerline. A house of up to 3,000 square feet needs two attached covered spaces. See the parking, setbacks and height page.

Ridgelines, Hillsides and the Airport

The Ridgeline Overlay Zone bars buildings within 100 vertical feet of a primary ridgeline, caps buildings at 28 feet, and needs a Planning Commission permit for two or more main buildings near a ridgeline. Land in the McClellan-Palomar Airport Influence Area goes through site plan review even for a building permit. See the ridgelines, hillsides and airport page.

Permits and Fees

The Director answers on completeness within 30 days, and appeals are due within 10 days. Public facilities fees, a park fee and, where a school district finds overcrowding, school fees are set by City Council resolution. VanPlex could not read the fee resolutions, so the page gives each fee's timing without amounts. See the permits and fees page.

How Current the Code Is

Municode's reader for San Marcos names the current edition "Supplement 14 Update 1." Its supplement history table ends with Ordinance 2026-1577 of March 10, 2026, which amended the Davia Village-MARC Specific Plan and is marked "Omit", meaning it was not added to the code text. The comprehensive zoning update, Ordinance 2025-1568 of December 9, 2025, is marked "Include" in Supplement 13, and its history notes appear on the residential, growth management and ridgeline chapters. Municode's ordinance records show nothing adopted after March 10, 2026.

Sources: Supplement history table and the history notes in Title 20, Title 19 and Chapter 17.44.

To find changes adopted after the code edition, VanPlex tried the City's own website. It answered the first request with a refusal (HTTP 403), and VanPlex sent no further requests to it. So this guide could not check City Council agendas, staff reports or ordinances adopted after March 10, 2026, including any local rules for SB 79, which took effect in San Diego County on July 1, 2026. Ask the Planning Division whether any zoning, subdivision or fee ordinance newer than Ordinance 2026-1577 is in force before relying on a rule here.

Where the Code, City Documents and State Law Differ

• Junior ADU owner occupancy. Always required in Sections 20.410.020(C) and 20.410.060(O); Government Code 66333(b) requires it only with shared sanitation.
• ADU fee line. No fees "on ADUs smaller than 750 square feet"; Government Code 66311.5 covers 750 square feet or less.
• ADU rental term. "30 days or longer" in the City code; "longer than 30 days" in Government Code 66323(e) and 66333(g).
• SB 9 unit size and height. 800 square feet, 16 feet and one story in Sections 20.470.030 and 19.44.050(L); Government Code 65852.21(b)(3) bars standards that do not apply uniformly in the zone.
• SB 9 denial grounds. Section 19.44.110(D) says "public health and safety of the physical environment"; the statutes allow denial for a specific, adverse impact on public health and safety.
• Ridgeline permits and SB 9. A Planning Commission permit for two or more main structures near a ridgeline, against the ministerial approval SB 9 requires.
• Within the code. R-1-7.5 at 6.0, 8.0 or "4.1-8.0" homes per acre; PRD density of "one (1) to eight (8)" or "four (4) to eight (8)"; R-2 listed under "Single-Family Zones" while it permits multifamily buildings; the Director or the Planning Commission as the ridgeline permit decider; and Planning Commission or both bodies for R-2 and R-3 building permits.

Limits of This Guide

Not read: the Zoning Map and the mixed use Regulating Plan; the General Plan and housing element; adopted specific plans such as San Elijo Hills; the McClellan-Palomar Airport Land Use Compatibility Plan; the ridgeline area maps; the fee resolutions (2024-9385, 2024-9386, 2024-9389), the park fee and inclusionary in-lieu fee resolutions; school district fees; and any City Council action after March 10, 2026. The City's website refused automated requests, so no City page is linked.

Ask the San Marcos Planning Division which zone, overlay and standards apply to your lot, which side of a code conflict it applies, what the current fees are, and whether an ordinance newer than March 10, 2026 is in force.

Where to Go Next

Related on VanPlex: the California ADU law page, the California impact fees page and the California SB 684 and SB 1123 page.

Frequently Asked Questions

Official sources (checked October 2026)